Re: [PATCH v6 2/6] arm64: ptrace: allow tracer to skip a system call

AKASHI Takahiro
2014-10-01T11:08:16Z

Will,

When I was looking into syscall_trace_exit() more closely, I found
another (big) problem.
There are two system calls, execve() and rt_sigreturn(), which change
'syscallno' in pt_regs to -1 in start_thread() and restore_sigframe(),
respectively.

Since syscallno is not valid anymore in syscall_trace_exit() for these
system calls, we cannot create a correct syscall exit record for tracepoint
in trace_sys_exit() (=> ftrace_syscall_exit()) and for audit in audit_syscall_exit().

This does not happen on arm because syscall numbers are kept in
thread_info on arm.

How can we deal with this issue?

-Takahiro AKASHI

On 08/27/2014 02:51 AM, Will Deacon wrote:
> On Fri, Aug 22, 2014 at 01:35:17AM +0100, AKASHI Takahiro wrote:
>> On 08/22/2014 02:08 AM, Kees Cook wrote:
>>> On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
>>> <takahiro.akashi@linaro.org> wrote:
>>>> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
>>>> index 8876049..c54dbcc 100644
>>>> --- a/arch/arm64/kernel/ptrace.c
>>>> +++ b/arch/arm64/kernel/ptrace.c
>>>> @@ -1121,9 +1121,29 @@ static void tracehook_report_syscall(struct pt_regs *regs,
>>>>
>>>> asmlinkage int syscall_trace_enter(struct pt_regs *regs)
>>>> {
>>>> + unsigned int saved_syscallno = regs->syscallno;
>>>> +
>>>> if (test_thread_flag(TIF_SYSCALL_TRACE))
>>>> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
>>>>
>>>> + if (IS_SKIP_SYSCALL(regs->syscallno)) {
>>>> + /*
>>>> + * RESTRICTION: we can't modify a return value of user
>>>> + * issued syscall(-1) here. In order to ease this flavor,
>>>> + * we need to treat whatever value in x0 as a return value,
>>>> + * but this might result in a bogus value being returned.
>>>> + */
>>>> + /*
>>>> + * NOTE: syscallno may also be set to -1 if fatal signal is
>>>> + * detected in tracehook_report_syscall_entry(), but since
>>>> + * a value set to x0 here is not used in this case, we may
>>>> + * neglect the case.
>>>> + */
>>>> + if (!test_thread_flag(TIF_SYSCALL_TRACE) ||
>>>> + (IS_SKIP_SYSCALL(saved_syscallno)))
>>>> + regs->regs[0] = -ENOSYS;
>>>> + }
>>>> +
>>>
>>> I don't have a runtime environment yet for arm64, so I can't test this
>>> directly myself, so I'm just trying to eyeball this. :)
>>>
>>> Once the seccomp logic is added here, I don't think using -2 as a
>>> special value will work. Doesn't this mean the Oops is possible by the
>>> user issuing a "-2" syscall? As in, if TIF_SYSCALL_WORK is set, and
>>> the user passed -2 as the syscall, audit will be called only on entry,
>>> and then skipped on exit?
>>
>> Oops, you're absolutely right. I didn't think of this case.
>> syscall_trace_enter() should not return a syscallno directly, but always
>> return -1 if syscallno < 0. (except when secure_computing() returns with -1)
>> This also implies that tracehook_report_syscall() should also have a return value.
>>
>> Will, is this fine with you?
>
> Well, the first thing that jumps out at me is why this is being done
> completely differently for arm64 and arm. I thought adding the new ptrace
> requests would reconcile the differences?
>
> Will
>